when is diaphragm action a necessity in floor deck systems

huangyhg

when is diaphragm action a necessity in floor deck systems
this one has always been a gray area to me.
you have a steel frame with joists to form a floor structure and you top it with trapezoidal deck and concrete topping.
in a seismic area i would utilize diaphragm action for better load distribution between frames. i would do this by using shear studs. this also gets me some composite action.
when seismic or wind load is not dominant i would simply brace the floor with horizontal bracing and i would ignore studs. the deck will be in place with the simple "nail " kind of fasteners. the braces distributes the load just like a diaphragm would at a lesser cost.
when no lateral load is available and i am relaxed with the stiffnesses of my sway frames and about sure that they have almost proportional sway stiffness, i would ignore bracing.
how do my friends approach this subject?
regards
ijr
check out our whitepaper library.
even without shear studs and composite   
i agree with taro - normally, with a metal deck with or without concrete, you do have a good diaphragm action if the deck is properly attached to the supporting steel and the lateral forces required for your diaphragm are checked.
the critical issue with flexible diaphragms come at the boundary where the lateral load is passed off to a supporting edge collector.  this is usually a fairly brittle connection (the weak/brittle link in the chain) and its very important that you design for a higher load capacity here (in fact:  the codes usually require you to do so).
also, a deck diaphragm without concrete is considered flexible (usually) and additional requirements for attaching load-bearing masonry or concrete walls is prescribed as well.
we almost never use additional bracing in the plane of the diaphragm when using metal deck diaphragms unless we can't get a particularly high shear to work with the deck we specify.